1. Field of the Invention
The present invention relates to an electronic braking device. In particular, the present invention relates to an electronic braking device for a vehicle such as, for example, for a motorcycle.
2. Description of the Background Art
A number of different designs for electronic braking devices, also known as by-wire type braking devices, are known. These types of braking devices detect the amount of the driver's brake operation, such as the pressure applied to a brake lever, and electrically perform braking according to the detection result. In this type of electronic braking device, the amount of brake operation is detected by a brake pedal sensor, and according to the detection result, an electric actuator activates a hydraulic modulator to apply a fluid pressure to a brake caliper and thereby brake the wheels. Such an electronic braking device is disclosed, for example, in Japanese published patent document JP-T No. 509753/2001.
The above electronic braking device must detect the amount of the driver's brake operation in an electrically wide range, and requires a sensor resolution which is high enough to cope with subtle changes in brake operation. For example, an excessively large input requires a wide dynamic range, and on a low-friction (μ) road surface such as a frozen road surface or manhole cover, a high resolution is required to cope with delicate brake operation. However, the use of a fluid pressure sensor that provides both a wide dynamic range and a high resolution results in a higher cost.
Therefore, the present invention provides an electronic braking device that easily generates a braking force depending on the amount of brake operation, in a range in which the amount of brake operation is small, including in the range just after the start of operation. As a result, the perceived feeling during braking is improved.
For vehicles such as motorcycles, there has been an electronic braking device in which a pressure sensor electrically detects a working fluid pressure corresponding to the amount of operation of a brake lever, brake pedal or the like, and according to the result of detection, an electric actuator actuates a hydraulic modulator to produce a fluid pressure and apply this fluid pressure to a brake caliper, thereby obtaining a braking force on wheels. This type of electronic braking device is disclosed, for example, in JP-A No. 264278/2000.
The above braking device has the following problem: it is possible that an unwanted excessive external force may be applied to the brake lever. As a result, the fluid pressure in the braking device excessively increases. A possible approach to coping with this excessive fluid pressure is to increase the tolerance of the fluid pressure sensor, but doing so would result in an unnecessary increase in cost.
Therefore, the present invention provides a braking device which protects the fluid pressure sensor from receiving a fluid pressure beyond its tolerance. By doing so, a breakdown of the fluid pressure sensor is prevented, and a change in its input/output characteristics is prevented. In addition, the fluid pressure sensor works rationally at low cost.